Reducing transport cost seems to be very important for the economy, to facilitate business, migration, tourism, etc.. For example, international tourism is a significant industry of many poor countries. (http://step.unwto.org/content/tourism-and-poverty-alleviation-1) Although increasing passenger density may make flights less safe, by an economy class syndrome or slower evacuation, etc., I think lower flight cost could actually reduce transport death as air is the safest way to travel by micromort per passenger-kilometer, and many people choose more dangerous methods of travel (car, bus, train, ship) because of the cost (http://www.numberwatch.co.uk/risks_of_travel.htm). Of course, the payload of any airplane is limited, so there may be limitations on the density of passengers. To reduce overall density, it is possible to sell less dense seats as well. (e.g. standing seat, supereconomy, economy, premium economy, partial recline business, full recline business, first, etc..) Also, to reduce payload, it is possible to not to allow check-in baggage and even making a cabin in the lower deck. Although my focus here is airplanes, this could be applied to bus, train, ship, etc. as well.

I would like to suggest several ways to increase passenger density:
1) Reducing seat pitch: It is possible to reduce seat pitch to as short as 28 inches. (https://www.seatguru.com/charts/shorthaul_economy.php)
2) Standing seat: Although this is similar to 1) it is much more radical. It is possible to reduce seat pitch even more by adopting standing seats, although regulations might not allow this.
3) Reducing seat width: It is possible to reduce seat width to as narrow as 16 inches. (ibid.) It is possible to make 13 seats abreast configuration on A380 main deck. (248-inch width of the main deck (http://www.airliners.net/forum/viewtopic.php?t=410157), 16 inches per seat, 20 inches per aisle.)
4) Reducing the number of the aisle of wide-body aircraft to one: This is not feasible right now due to regulations such as 14 CFR 25.817 (https://www.law.cornell.edu/cfr/text/14/25.817), which limits the number of seats abreast to 6 in single-aisle aircraft.
5) Lower deck cabin: Lower decks (cargo deck) of airplanes are almost as big as the passenger deck or in case of A380, almost as big as the upper deck. Some planes have crew rest or lavatory at lower deck. (see, e.g., http://www.telegraph.co.uk/travel/news/Who-needs-windows-The-plane-where-passengers-sit-in-hold/)